Protective coatings containing the curing products of aminoplast resins and polyvalent alcohols are known to improve the abrasion resistance and solvent resistance of thermoplastic substrates. For example, U.S. Pat. No. 4,552,936 to Moore generally describes a protective coating composition containing a polycaprolactone polyol and an aminoplast derivative. As another example, U.S. Pat. No. 4,714,657 to Quinn et al. describes protective coatings derived from polyester polyols and aminoplast derivatives. Another example is provided by U.S. Pat. No. 4,913,974 to Moore et al., which describes coated articles comprising a thermoplastic substrate and a surface coating that is the reaction product of a melamine compound, a polyol, and a multimeric benzotriazole compound. Increasingly demanding product applications have created a need for coated thermoplastic sheets with an improved balance of abrasion resistance, solvent resistance, weatherability, and formability.